Increased concentrations of octopamine and tyramine in brain tissue and blood of patients and experimental animals with hepatic encephalopathy have been reported. The capability of these aromatic amines to interfere with normal catecholaminergic neurotransmitter function in both the central and peripheral nervous system is well described. Further investigation into the metabolism of octopamine and tyramine and their relationship to the pathophysiology of hepatic encephalopathy and other mental disorders should provide the knowledge needed to effectively treat these diseases. However, the currently available methods to measure octopamine and tyramine in tissue and biological fluids by radioenzymatic or radioimmunoassays are costly, time-consuming and/or insensitive. This proposal is directed toward developing an inexpensive, rapid and sensitive I125 radioimmunoassay for octopamine and tyramine. The radioimmunoassay is based on our previously successful published technique for a synephrine I125 radioimmunoassay. This method utilizes high specific activity (greater than 1000 Ci/mmole) 3-iodosynephrine I125 synthesized by the chloramine T oxidation method and antisera produced against a BSA conjugate of synephrine. The assay is simple, rapid, inexpensive and sensitive. We expect similar assay characteristics for octopamine and tyramine, since as monohydroxylated phenylethylamines, they are the same class of compounds as synephrine. The I125 radioimmunoassay can then be applied as a more easily available tool to many more investigations in the study of hepatic encephalopathy and other mental disorders.